A substantial amount of research has been performed to develop a reliable control for an electric motor or machine for use in critical service applications, such as in medical, military, and transportation applications. For example, in transportation applications where high reliability is required to maintain safety, hybrid and electric vehicles have begun to be widely used. In order to maximize the fault tolerance capabilities and minimize the cost of such electric devices, including electric transportation systems, the use of multi-phase motor systems have been investigated. Due to their high number of phases, multi-phase motor systems are desirable due to their ability to continue to operate and provide a significant amount of torque even in the case when one or more phases are lost or made operational due to a fault condition.
Among the many types of multiphase motors, permanent magnet assisted synchronous reluctance motors (PMa-SynRM) have been considered as one of the most promising motor technologies due to its many benefits, including its robust control and low-cost design. In particular, the five phases of the PMa-SynRM utilizes the features of both synchronous reluctance machines (RSM) and permanent magnet synchronous machines (PMSM) to improve its torque producing characteristics. Furthermore, in PMa-SynRMs, the number of permanent magnets that are used is reduced, as compared with other types of PMSMs, and as a result, the overall cost of the PMa-SynRM is reduced. Moreover, due to the presence of reluctance torque, the control strategy of PMa-SynRM motors can also be enhanced to optimize the torque that is able to be provided in the event of a fault condition.
As such, it would be desirable to develop a control system that can further enhance the operation of the multi-phase permanent magnet assisted synchronous reluctance motor (PMa-SynRM), so as to include various operational advantages, including a reduction of torque pulsation, reduction of stator current per phase without increasing the voltage per phase, improving the torque per ampere, reducing the DC link current harmonics, while also offering higher reliability. These characteristics support the use of multi-phase motors/machines as an excellent candidate for greater fault tolerant operation in vehicular and military applications.
In order to sustain reliable operation of such PMa-SynRM motors, control methods are required to respond in an appropriate manner when the motor experiences a detected fault, while maintaining an acceptable level of motor control performance. As such, several strategies have been previously evaluated for reliable fault tolerant control of such multi-phase machines or motors. For example, some control techniques utilize a control system that includes redundant phases in the inverter side or additional machines/motors in parallel to continue motor operation when one motor experiences a fault, which is not cost effective. In addition, significant research has taken place to develop a control scheme that maintains the same amount of total current prior to a motor fault and after the occurrence of a motor fault. For example, one strategy that has been considered involved providing more phase current (200%-300% for two phase faults) in the healthy phases, while disconnecting the faulty phase. However, by increasing the phase current significantly may cause critical machine/motor parameters of the PMa-SynRM to change, such as inductance, which is prone to going into the saturation region. Furthermore, the operation of the PMa-SynRM motor with such increased phase currents may lead to decreased torque, lowered efficiency, and increased operating temperature, etc. Also, the higher amount of phase current that is to be applied to the motor requires it to have a higher rated design, which may not be cost effective. Moreover, under such conditions, the motor would not be capable of being operated for a long duration. Still, other strategies have been evaluated to reduce the torque ripple of the PMa-SynRM while considering the very low average torque (˜83% reduction) under a two-phase fault condition. However, the application of this technique is limited where priority is given to obtain higher torque.
Therefore, there is a need for a fault tolerant control system for a permanent magnet assisted synchronous reluctance motor (PMa-SynRM), whereby the amplitude (less than about 150%) and angle of the phase currents applied to the motor are optimized using a computer simulation, such as a MATLAB simulation, to maximize the torque output by the motor. In addition, there is a need for a fault tolerant control system for a permanent magnet assisted synchronous reluctance motor (PMa-SynRM) that adjusts the phase advance (i.e. the offset between rotor position and the stator current reference) to provide optimal and sustainable torque when the motor is under various fault conditions. Furthermore, there is a need for a fault tolerant control system for a permanent magnet assisted synchronous reluctance motor (PMa-SynRM) that enables various operational advantages, including a reduction of torque pulsation, a reduction of stator current per phase without increasing the voltage per phase, an improvement of the torque per ampere, a reduction of the DC link current harmonics, and an increase in reliability.